E and D String Relationships on the Fretboard
Question: In the first example in the 2nd fretboard lesson, the relationship between the low E, high E, and d strings are shown. I get that the two E strings have the exact same notes but I don't understand what you said about the D string.
"The other related string in that diagram was the D string. All you need to do is look at its position in relation to the high or low E string - it's 2 frets higher. So if you fretted the D string at fret 5 it would be the note G because the E string in its related position would be at fret 3, which you'd know is also G."
If you fret the D string at the 5th fret, it would be G because the E string is at the 3rd fret??? Are you just saying that playing two frets higher yields the same note?
Answer

Yes, that's all I'm saying. I wanted to highlight the importance of seeing these relationships between the strings.
If you've identified a low E string root note in a chord or scale, for example, you should know that the D string, two frets higher, is the same note and therefore will provide a higher root note for that chord/scale.